LET’S start with the good news: Cameron Diaz and Ashton Kutcher actually have combustible chemis try in “What Happens in Vegas,” something that occurs much less often than you’d expect in contemporary romantic comedies.

That and the fact that they’re gorgeous physical specimens (something the movie takes great pains to showcase) and basically likable screen personalities may well be enough to satisfy legions of audience members, not to mention the bookkeepers at our sister company 20th Century Fox.

The bad news is that director Tom Vaughan (“Starter for 10”) goes out of his way to encourage the stars’ tendency to mug. Laughs are few and far between in the innuendo-laden script attributed to Dana Fox, who’s also responsible for the reprehensible “The Wedding Date.”

For an idea of the level of wit, several gags are built around the name Dick Banger, which belongs to Diaz’s boss. He’s played by Dennis Farina, and his main function is to give an unprintable nickname to Kutcher’s character.

Diaz portrays the ironically named Joy, an uptight Manhattan stockbroker who has fled to Las Vegas after being dumped by her fiancé in front of all her friends at his surprise birthday party.

There she meets Jack (Kutcher), a womanizing man-boy of a cabinet builder who’s just been fired by his boss (Treat Williams), who also happens to be Jack’s fed-up father.

Hate at first sight turns into alcohol-fueled lust and a quickie Vegas wedding, which Jack’s lawyer pal Steve (Rob Corddry) promises to have annulled over breakfast the next morning.

Unfortunately, there’s a $3 million slot-machine jackpot involved, and the extremely contrived plot requires Judge Whopper (Dennis Miller) to sentence the newly married couple to “six months hard marriage” while he and a counselor (Queen Latifah) try to sort things out.

Egged on by Steve and Joy’s BFF (Lake Bell), the couple, newly cohabitating under the Brooklyn Bridge, try to drive each other crazy and into the arms of others. This involves recruiting “sluts,” removing toilet seats and trying other tired gambits that are telegraphed well in advance and dragged out far past the point of diminishing returns.

Screenwriter Fox claims “What Happens in Vegas” was inspired by Britney Spears’ first marriage, but I’d find it hard to believe she hasn’t seen “Laws of Attraction,” a terrible Pierce Brosnan-Julianne Moore romantic comedy set mostly in Ireland with a virtually identical plot. Didn’t work then, either.